Several aspects of mitochondrial monoamine oxidase activity in biogenic amine catabolism are being investigated: 1) Large dense deposits are formed in or on the outer membrane when benzylamine or tyramine is oxidized by isolated rat liver mitochondria. The chemical nature of these deposits is being investigated to determine if they may be condensation products similar or identical to those reported to occur under certain pathological conditions. 2) The formation of these products in normal rat liver and brain mitochondria and tissue is being compared to similar products formed in experimentally induced alcoholic animals. Alkaloid condensation products formed between aldehyde(s) and amine(s) are reported to be candidates for the compounds responsible for addiction to alcohol and possibly other drugs. 3) The possible role of mitochondria in the uptake and storage of serotonin by isolated platelets is being studied since these processes are ATP-dependent. The nature of MAO (A vs B) associated with mitochondria isolated from platelets is under investigation as well. This differentiation is based on substrate and inhibitor specificities. 4) Final conclusions concerning the validity of interpretations suggesting that mitochondrial MAO exists as multiple forms (isozymes) awaits isolation and purification of distinct proteins. As an approach toward solving this problem, various tissues from different sources are being assayed with differentiating inhibitors and substrates. On this basis some tissues appear to have only a single activity, A or B. Using this information the different activities will be isolated, purified, and characterized; since only one of the two activities is present, purification procedures should be relatively free of the problem of contamination with the other. 5) Evidence has been obtained which indicates that mitochondrial MAO may shuttle electrons into the respiratory chain of the inner mitochondrial membrane. Data supporting this possibility have been obtained from studies using respiratory inhibitors and by measuring difference spectra of respiratory carriers during oxidation of MAO substrates by intact mitochondria, by frozen-thawed mitochondria, and frozen-thawed, washed membranes.